During certain periods of differentiation, embryonic tissues become particularly sensitive to the influence of factors in their environment. One example is the association of a rare form of vaginal cancer in young women with the ingestion of the synthetic estrogen, diethylstilbestrol (DES), by their mothers during pregnancy. This project seeks to determine whether there is a period in the development of the female rat when mammary tissue can be affected by hormones such as DES. The effects of prenatal exposure to a hormone will be measured as alterations in the incidence, growth characteristics, histology and estrogen binding capacity of mammary tumors arising in rats after postnatal treatment with 7, 12-dimethylbenz(a)anthracene (DMBA). Demonstration of effects on mammary tumor incidence after prenatal hormone exposure in rats could be a significant warning that an increased risk for the development of breast cancer exists in young women whose mothers took DES during pregnancy.